1. Field of the Invention
This invention relates to magnetic resonance (MR) measurement of shear rates of moving materials and more specifically to MR mapping of the velocity of a moving material.
2. Description of Related Art
Shear rate is defined as the change in velocity of a material with respect to position. It is an important parameter in indicating regions of great or little velocity. In flowing materials, such as liquids flowing through metal tubes, shear rate influences the rate of corrosion. By localizing regions of extremely high or low shear rates, portions of a tube wall which may possibly fail could be identified.
Shear rate is also related to the of development of arteriosclerotic disease in mammals. Locations which have low shear rates, are more apt to develop plaque and blockage characteristic of this disease.
A traditional fluid flow analysis method, known as ink streamlining, requires introducing a contrast agent into a flowing fluid and observing the motion of the contrast agent.
Another method of measuring shear rate employs laser Doppler methods. This requires a laser beam to be reflected from particles suspended in the material which is to be measured, and determining the displacement of each particle over a short interval thereby indicating the velocity of the material at the chosen location.
Both of these methods are invasive, or destructive and require direct access to the material being tested. If the material is inside a tube or deep within a living subject, these methods will not be useful. Furthermore, they are not suited for in-vivo, or non-destructive testing applications.
Since velocity is a vector quantity, it can be expressed as the stun of three mutually orthogonal component vectors. Each of these components, in turn, can be measured with respect to three mutually-orthogonal spatial dimensions to give a total of nine different shear rate measurements. Existing techniques can be used to measure some of these shear components, but detection of all components is difficult or impossible in most situations.